Traction drive machines having small internal combustion engines, such as powered machines used for lawn care, commonly employ exhaust systems to convey the exhaust gas from the engine cylinders to the ambient environment. Small internal combustion engines are typically defined as engines having 25 horsepower or less.
Small engines have higher concentrations of exhaust gas constituents requiring conversion. This is a result of the richer air:fuel ratios required in small engines for successful operation and engine cooling. The compact size of the exhaust catalyst, the high specific throughput (about 10 times higher than automotive), and the high concentration of emission constituents result in high heat generation rates and high catalyst temperatures.
These exhaust systems typically do not include means to control the effects of “off nominal” conditions of engines. Off nominal conditions can be described as when one or more cylinders are not functioning properly (or optimally) such that the air:fuel exhaust gas mixtures entering the exhaust system are combustible mixtures and create excessively high exhaust gas and system temperatures if ignited. Sources of off nominal conditions include ignition misfiring, air:fuel cylinder-to-cylinder imbalance, and mechanical malfunction of an intake or exhaust valve. Sources of ignition include hot exhaust catalysts and exhaust gasses from a nominal cylinder.
Therefore, off nominal conditions are very dangerous when the exhaust gasses from multiple cylinders are allowed to mix because the high exhaust gas and system temperatures from a nominal cylinder could ignite an unburned fuel mixture from an off nominal cylinder and cause unintended combustion and flames. For example, a V-twin air-cooled two cylinder engine with a dual-inlet single-outlet exhaust with catalysts normally has an exhaust gas temperature of 1350° F. However, if one sparkplug is caused to miss-fire, an unignited fuel:air exhaust gas mixture enters an exhaust chamber, mixes with ignited exhaust gasses from nominal cylinders, and ignites, which causes the exhaust gases to increase 1020° F. in 10 seconds to 2370° F. Further, off nominal conditions increase the risk of a meltdown of the catalyst substrate, which can result in catalytic deactivation and severe exhaust restriction.
The exhaust temperatures which occurred during off nominal conditions in the scenario described above would be even higher if small engine designers attempted to achieve catalyst efficiencies approaching those of automotive catalysts. The high exhaust gas concentrations and high space velocities produced by a higher efficiency catalyst could create even higher heat loads and temperatures. Thus, in the scenario described above, it is necessary to limit the initial catalyst efficiency to protect the engine and exhaust system.
It is known that some engine fuel management systems include oxygen sensors or temperature sensors that intend to limit the effects of off nominal conditions found during operation of engines, but these are typically expensive and create a machine control issue by reducing the overall engine power as quickly as possible when an off nominal condition is detected.
Accordingly, a need exists for an inexpensive exhaust system that reduces the dangers of off nominal conditions.